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Java advanced examples and exercises

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Java advanced examples and exercises

Enums public class Main { public static void main(String[] args) {

Season[] values = Season.values();

for (Season s: values) {

System.out.println(s);

Scanner scanner = new Scanner(System.in);

System.out.print("Which season do you like? ");

String myseason = scanner.nextLine();

Season favorit = Season.valueOf(myseason.toUpperCase());

System.out.printf("So you like %s?%n", favorit);

System.out.printf("It is number %d in the list%n", favorit.ordinal()); enum Season {SUMMER, AUTUMN, WINTER, SPRING};

Generics

Generics01

We have to add @SuppressWarnings because we are using a collection class without generics (normally we do not do that anymore). Because we are not using generics the arraylist is not typesafe (can contain strings and integers) public class Main { @SuppressWarnings("unchecked") public static void main(String[] args) { @SuppressWarnings("rawtypes")

ArrayList list1 = new ArrayList();

list1.add("Elvis"); // add a String list1.add(42); // add a number list1.add(new Person("Elvis", 42)); // add a person for (Object o: list1) {

System.out.println(o);

class Person{ private String name; private int age; public Person(String name, int age) { this.name = name; this.age = age; public String getName() { return name; public int getAge() { return age;

Java advanced examples and exercises

2

Generics02

When we declare the arraylist as an arraylist of Strings (ArrayList) it is impossible to add integers: public class Main { public static void main(String[] args) { ArrayList list1 = new ArrayList(); list1.add("Elvis"); //list1.add(42); //not possible //list1.add(new Person("Elvis", 42)); //not possible for(String s:list1) {

System.out.println(s);

Generics03

We deciare our own generic Trap-class:

public class Trap { private E prisoner; public void catchPrisoner(E prisoner) { this.prisoner = prisoner; public E releasePrisoner() {

E tmpPrisoner = this.prisoner;

this.prisoner = null; return tmpPrisoner; /*public String getPrisonerName() { * //not possible, not every class has a getName() method * return prisoner.getName(); And now we can declare a Trap for certain types of animals: public class Main { public static void main(String[] args) {

Trap bearTrap = new Trap();

Bear baloo = new Bear("Baloo");

Duck donald = new Duck("Donald");

bearTrap.catchPrisoner(baloo); //bearTrap.catchPrisoner(donald); //error: bearTrap cannot catch ducks

Animal a = bearTrap.releasePrisoner();

System.out.printf("%s is released", a.getName());

class Animal { private String name; public Animal(String name) {

Java advanced examples and exercises

3 this.name = name; public String getName() { return name; class Bear extends Animal{ public Bear(String name) { super(name); class Duck extends Animal{ public Duck(String name) { super(name);

Generics04

When can also declare a Trap that can only be used for Animal (or derived from Animal) objects: public class AnimalTrap { private E prisoner; public void catchPrisoner(E prisoner) { this.prisoner = prisoner; public E releasePrisoner() {

E tmpPrisoner = this.prisoner;

this.prisoner = null; return tmpPrisoner; public String getPrisonerName() { //Every prisoner is an Animal with a getName() method return prisoner.getName();

Arrays and Covariance(cocontravariance)

Covariance: reading also works for subclasses, writing covariant does not work in Java. public class CoContraVariance { public static void main(String[] args) { //Covariant arrays reading String[] K3_old = {"Karen", "Kristel", "Kathleen"}; String[] K3_less_old = {"Karen", "Kristel", "Josje"}; if (equalArrays(K3_old, K3_less_old)){

System.out.println("Arrays are equal");

}else {

System.out.println("Arrays are not equal");

//ArrayStoreException at runtime while writing doSomethingWithArray(K3_old);

Java advanced examples and exercises

4 //works CoVariant: also OK for child classes public static boolean equalArrays(Object[] o1, Object[] o2) { //skip null checking if (o1.length != o2.length) return false; for(int i=0;iObject o = objects[0];

System.out.println("First object is "+o);

//writes give error at runtime //objects[0] = LocalDate.now(); Generics: Covariant (extends) reading and ContraVariant (super) writing(cocontravariancegenerics) public class CoContraVarianceGenerics { public static void main(String[] args) {

Shelter birdshelter = new Shelter<>();

saveAnimalInShelter(birdshelter); printAnimalFromShelter(birdshelter); //writing public static void saveAnimalInShelter(Shelter shelter) { shelter.setAnimal(new Chicken("Tweety")); //reading public static void printAnimalFromShelter(ShelterAnimal> shelter) {

Animal a = shelter.getAnimal();

System.out.println(a);

class Animal { public String getName() { return name; public void setName(String name) { this.name = name; private String name;

Java advanced examples and exercises

5 public Animal(String name) { this.name = name; @Override public String toString() { return "Animal{" + "name='" + name + '\'' + class Bird extends Animal{ public Bird(String name) { super(name); @Override public String toString() { return "Bird{" + "name='" + getName() + '\'' + class Chicken extends Bird{ public Chicken(String name) { super(name); @Override public String toString() { return "Chicken{" + "name='" + getName() + '\'' + class Shelter { private T animal; public T getAnimal() { return animai; public void setAnimal(T animai) { this.animai = animai;

Collections

JavaCollection01

A class (like ArrayList) that implements Iterable can be iterated over. When it implements Collection

collection to Strings. When we declare a Collection variable of generic type String it can only be implemented by a class (ArrayList) of type String. From Java 7 on, we can use the diamond operator and leave out the

Java advanced examples and exercises

6 public static void main(String[] args) { //Collection building = new ArrayList(); // From Java 7 onwards

Collection building = new ArrayList<>();

building.add("Elvis"); if(building.contains("Elvis")) {

System.out.println("Elvis is in the building");

} else { System.out.println("Elvis has left the building"); if (building.isEmpty()) {

System.out.println("The building is empty");

} else { System.out.println("There is someone in the building"); building.add("Priscilla");

Iterator it = building.iterator();

while (it.hasNext()) {

String name = it.next();

System.out.println(name);

//When a class has an iterator, we can use a for each for (String s: building) {

System.out.println(s);

building.clear();

JavaCollection02

Collections can be converted into streams (Java 8). We can filter streams, sort them, use a foreach public static void main(String[] args) { Collection company = new ArrayList(); company.add(new Employee("Elvis", "Entertainment")); company.add(new Employee("Priscilla", "Accounting")); company.add(new Employee("Colonel Parker", "Management")); company.add(new Employee("Lisa Marie", "Accounting")); //Java 8 company.stream() .filter(e -> e.getDepartment().equals("Accounting")) .forEach(e -> System.out.println(e.getName())); public class Employee { private String name, department; public Employee(String name, String department) { this.name = name; this.department = department; public String getName() { return name; public String getDepartment() {

Java advanced examples and exercises

7 return department;

Javacollection03

A set contains unique items(the same string cannot be added twice) public static void main(String[] args) {

Set items = new HashSet();

items.add("Elvis"); boolean succeeded = items.add("Elvis"); // false if (!succeeded) {

System.out.println("There is only one Elvis");

System.out.println("The set contains " + items.size() + " elements");

Javacollection04

Two employees are the same when their equals()-method says they are the same or when they refer to the same object in memory public static void main(String[] args) {

Set items = new HashSet();

items.add(new Employee("Elvis")); boolean succeeded = items.add(new Employee("Elvis")); //true!!! if (succeeded) { System.out.println("There is an Elvis impersonator here");

Iterator it = items.iterator();

while(it.hasNext()) {

Employee e = it.next();

if (e.getName().equals("Elvis")) it.remove(); System.out.println("The set contains " + items.size() + " elements"); public class Employee {quotesdbs_dbs9.pdfusesText_15

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